1. Field of the Invention
The present invention relates to a bias voltage control device for use in an electrophotographic duplicator employing a temperature-dependent sensitive material (hereinafter referred to as sensitive material) whose static charge characteristic varies with temperature variation, and more particularly to a device capable of automatically changing the bias voltage being applied to a developing electrode in accordance with the temperature variation of the sensitive material.
2. Discussion of the Prior Art.
Generally, with the development with charged toner of a static latent image occurring on a photoconductor or other charged insulating material, a problem, known as white dropout, arises. In particular, the central portions of large-image regions or of thick characters fails to be developed with the toner. This phenomenon results from the electric field of the charged latent image being concentrated at its end portions, the field intensity being low in the center of the image region whereby uniform deposition of toner is difficult. To prevent white dropout, developing electrodes have been used heretofore. Here, the term "developing electrode" is defined to include an electrode plate in a cascade developing unit and a developing roll in a magnetic-brush developing unit.
The developing electrode is disposed in proximity to the region to be developed so as to generate an electric field of uniform intensity over the entire region so that toner is also directed by the developing electrode to the central portions of large-image regions or thick characters, and effect uniform development thereof. Usually the bias potential applied to the developing electrode is higher than the potential of the image background region to prevent deposition of toner onto the background region. Further, the bias potential is sufficiently lower than the potential of the image region.
As described above, it is possible, with a developing electrode, to achieve satisfactory development even of the central portions of large-image regions or thick characters. However, since the bias voltage applied to the developing electrode is usually set to a fixed value, satisfactory development can be expected only when the static charge characteristic of the sensitive material is stable with respect to variations of ambient conditions. If the sensitive material used has a static charge characteristic that varies with ambient conditions, the developing electrode, to which a fixed bias potential is applied, may conversely bring about deterioration of image quality since as stated above, the bias potential should be higher than the background potential. However, if the background potential increases the developing electrode potential, toner will undesirably deposit on the background region.
In FIG. 1, curve 1 graphically represents the relationship between the temperature of a temperature-dependent sensitive material and the potential of the background region on the sensitive material. It is obvious from this graph that the potential of the background region on the sensitive material rises as the sensitive material temperature decreases in accordance with ambient temperature variation. If a large temperature drop occurs, the background region potential increases above the developing electrode potential. As a result, toner is undesirably deposited on the background region to cause conspicuous stain. This phenomenon is generally called "fog". One of the means to eliminate fog is to apply a high bias potential to the developing electrode even under normal ambient conditions, in anticipation of a temperature decrease of the sensitive material and a corresponding potential rise in the background region. However, this decreases the potential difference between the developing electrode and the image region potential. Thus, a high-concentration developer is required for a desired image density. As a result, there is an undesired increase of toner comsumption and machine contamination.
Moreover, the image region potential varies as the number of copies increases. If the image region potential is relatively low as described above, the image potential variation becomes large whereby the image density is liable to change even when a developing agent of the same concentration is used.